1. Field of the Invention
The present invention relates to an optical element which changes optical density by making the use of electromotive force corresponding to the intensity of electromagnetic wave and camera unit comprising same.
2. Background of the Related Art
An element which responds to electromagnetic wave to change optical density finds a wide application. As a material capable of responding to electromagnetic wave to change optical density, i.e., control the transmission or reflection of light there may be used a photochromic material or electrochromic material.
A photochromic material changes its optical density when irradiated with light. The photochromic material is applied to sunglass, ultraviolet checker, printing-related material, processed fiber product, etc.
An electrochromic material changes its optical density upon the inflow and outflow of electron. The electrochromic material is applied to automobile anti-glare mirror, vehicle window material, etc.
One of the uses of such an optical density-changing element is a picture-taking system such as camera. For example, a film with lens has recently been widely spread as a ready-to-use camera unit requiring no loading with film because of its convenience. In order to enhance the utility of such a film with lens, development has been made to use a high sensitivity film in this camera unit. However, the related art film with lens featuring convenience was not provided with a mechanism for adjusting exposure. Therefore, when picture is taken by a film with lens comprising a high sensitivity film under bright conditions, there occur many cases where excessive exposure causes whitening of image and hence failure in picture-taking. Then, a film with lens which employs AE control system involving photometry during picture-taking to allow automatic switching of stop according to the amount of light taken has been put on the market. In this arrangement, the frequency of occurrence of failure in picture-taking due to overexposure has been drastically reduced.
As an approach for realizing such a “light adjusting filter” capable of adjusting the amount of light incident on the photographic light-sensitive material according to the amount of light taken simply at reduced cost there has been proposed a film with lens comprising the aforementioned photochromic material (see, e.g., JP-A-5-142700, JP-A-6-317815 and JP-A-2001-13301). More specifically, the photochromic material undergoes color development or shows an optical density rise when irradiated with light having a certain wavelength and undergoes discoloration or shows an optical density drop when suspended in irradiation with light, heated or irradiated with light having different wavelength. As such photochromic materials there have been known silver halide-containing inorganic compounds and some organic compounds. It has been proposed that when a filter made of a photochromic material disposed on the optical axis is allowed to undergo color development/discoloration according to the amount of incident light, light adjustment can be made.
However, it is usual that a photochromic material takes about 1 minute to undergo color development and scores of minutes or more to undergo discoloration (see, e.g., Solid State and Material Science, 1990, Vol. 16, page 291). Thus, a photochromic material can be difficultly used as a system for adjusting light taken.
On the other hand, as a material capable of undergoing faster color development/discoloration there may be used the aforementioned electrochromic material. More specifically, an electrochromic material shows an optical density rise when subjected to application of a voltage to receive or discharge electron and shows an optical density drop when electron moves in the direction opposite that of density rise. It is known that some metal oxides and organic compounds have such a nature. The combined use of such an electrochromic material, an electric source and a sensor for measuring the amount of light taken makes it possible to realize a “light adjusting filter” for adjusting the amount of light incident on a photographic light-sensitive material according to the amount of light taken.
A light adjusting system has been proposed comprising a laminate of such an electrochromic material and a solar cell which responds to light to generate electromotive force (see, e.g., JP-A-9-244072). This system, too, can be expected to perform automatic light adjustment according to the amount of light taken. However, such a laminated structure of solar cell and electrochromic material as proposed above is disadvantageous in that part of light rays transmitted by the electrochromic material layer is unavoidably absorbed by the solar cell. Therefore, this proposal is not suitable for a system requiring maximum utilization of transmitted light in a scene requiring no light adjustment such as camera-related optical element.
On the other hand, it is reported that such an electrochromic material shows an enhanced response when used adsorbed by a porous titanium oxide or antimony-doped tin oxide layer (see, e.g., JP-T-2000-506629, JP-T-2003-511837, PROCEEDINGS SPIE—The International Society for Optical Engineering XIV, 1995, Vol. 2,531, page 60 and Journal of Physical Chemistry B, 2000, Vol. 104, page 11,449).
As a light adjusting filter for picture-taking there was used an electrochromic element comprising a metal oxide as disclosed in Journal of Physical Chemistry B, 2000, Vol. 104, page 11,449. As a result, ill-balanced color and haze rise occurred, demonstrating that this electrochromic element is not of use. For this purpose, an electrochromic element having a higher light transmittance is desired. In order to use such an electrochromic element as an “automatic light adjusting filter”, an electric source for driving the electrochromic element is needed. This electric source is particularly required to generate a proper electromotive force according to the amount of light taken and properly adjust the operation of the electrochromic element.
Metal oxides which have heretofore been used for this purpose leave something to be desired in light transmittance as mentioned above. It has been desired to give solution to this problem.